Couple of years ago I tested and made music with 12 different music editors.
Once I tried SF2, I felt I'm at home and it immediately clicked with me. This means, to my workflow and brain-capacity SF2 is the closest and best thing. Which, in your case might be different.
I'd suggest you to give GT2, SF2, CC and SID-Wizard a testrun, and not only a testrun but use them for a couple of weeks. Then you'll have a feel for one of them, that's gonna be your tool for C64 music. See, if you settle for one that is not right but people said it's the best... well, then you will feel frustration every time you start making music.
All of the above mentioned editors have great documentation (also, a SF2 tutorial from me is long due and it's also WIP). If you have any question, we surely can find you the best person to help let it be about GT2, SF2 or any of the music tools.